The disclosure relates generally to a photomask and a method of fabricating a pattern using the photomask, and, more particularly, to a photomask for forming a line pattern with a low density and a method of fabricating the pattern using the photomask.
As integration degree rapidly increases, limitations in photolithography technology for fabricating a pattern have reached a serious level. Accordingly, various methods have been developed to overcome the aforementioned limitations, and one of which is to use an off-axis illumination system or an asymmetric illumination system.
FIG. 1 is a layout diagram illustrating a photoresist layer pattern for forming a line/space-type gate pattern as an example of a pattern for a semiconductor device. As illustrated in FIG. 1, to form a line/space-type gate pattern, an exposure and development process by photolithography technology is first implemented to form photoresist layer patterns 101, 102. Next, an etch process on an exposed portion of a lower layer 111, 112 is implemented using the photoresist layer patterns 101, 102 as an etch mask to form gate patterns corresponding to the photoresist layer patterns 101, 102. In general, in a semiconductor device having a cell region and a peripheral region, and particularly in a dynamic random access memory (DRAM) device, the gate patterns are arranged with a narrow spacing between two adjacent gate patterns in an X-axis direction in the cell region CELL, whereas the gate patterns are arranged with a wide spacing between two adjacent gate patterns in an X-axis direction in the peripheral region PERI.
FIG. 2 illustrates a dipole illumination system, which is an example of an off-axis illumination system used in an exposure process for forming the photoresist layer patterns of FIG. 1 and an example of an intensity distribution of light transmitted through a photomask. As illustrated in FIG. 2, a dipole illumination system 210 has light transmitting parts arranged symmetrically in an X-axis direction. When implementing the exposure using this dipole illumination system 210, in the cell region, as shown in the graph 220, a difference between an intensity of light transmitted through a light shielding region 231 of the photomask 230 and an intensity of light transmitted through a light transmitting region 232 is significant in the X-axis direction, and a high resolution in the X-axis direction can thus be obtained.
However, in the peripheral region PERI having a relatively low density or an isolated shape, a zeroth order diffraction light and a first order diffraction light transmitted through the photomask are not symmetrical any more. Therefore, because slight variations in a process variable, such as a focus offset, have a large influence on a pattern profile, a pattern collapse as indicated by 310 and 320 in FIGS. 3A and 3B, respectively, may be generated in extreme conditions.
This will be described with reference to FIGS. 4A through 4C which illustrate symmetry of light according to a density of the pattern. First, as illustrated in FIG. 4A, when a pitch is smallest, for example, when the pitch (denoted as “P” in FIG. 1) is 150 nm, zeroth order lights 411, 431, a negative first order light 421 and a positive first order light 441 forms a suitable symmetrical structure. In particularly, the negative first order light 421 and the zeroth order light 431, which should interfere with each other, are arranged generally symmetrically along an X-axis direction. On the other hand, as illustrated in FIG. 4B, when the pitch P is increased to 200 nm, the negative first order light 421 and the positive first order light 441 are moved toward the middle, and, thus, the negative first order light 421 and the zeroth order light 431, which should interfere with each other, show a degraded symmetry along the X-axis direction.
Furthermore, as illustrated in FIG. 4C, when the pitch P is increased to 300 nm, the negative first order light 421 and the positive first order light 441 are moved to the middle, and, thus, the negative first order light 421 and the zeroth order light 431, which should interfere with each other, are not symmetrical with each other anymore along the X-axis direction.
As described above, when implementing the exposure for forming a line-type pattern (101, 102 of FIG. 1) using an asymmetric illumination system having directivity such as a dipole illumination system (210 of FIG. 2), a sufficient resolution along the X-axis direction is obtained and the profile of the patterns 101, 102 is not easily changed by the variation in the process variable such as the focus offset in the cell region CELL having a high pattern density. However, in the peripheral region PERI having a low pattern density, the symmetry of the zeroth order light 431 and the negative first order light 421 is degraded and the profile of the patterns 101, 102 is changed even with a slight variation in the process variable, such as the focus offset, thereby resulting in the generation of pattern collapse.